One main reason to use managed switches is just to block multicast traffic from spewing forth across a larger network. For that, you need to enable IGMP snooping on the switches as a bare minimum
The ability of almost all managed switches now to handle a ring network (STP - Spanning Tree Protocol) is nice too, as it can prevent a total network crash from plugging a switch into itself.
Security is another reason in more and more environments... managed switches usually allow you to lock ports to MAC ID's, and shut off unused ports to prevent unauthorized network taps.
Remote Diagnostics can also be handy, and most managed switches have a web page that can give you information on what ports are active, if there are any detected errors on various ports, etc. Most also have the ability to 'Mirror' any port to another, for advanced diagnostics or protocol snooping.
While I prefer using all managed switches, in some cases it really doesn't make sense. I have a few lines here that have a total of 7 or 8 switches each. On those, the only managed switch is the one that connects the line to the entire plant side network. The rest are just standard switches.
